One outdoor unit. Heating, cooling and hot water.
Air-to-water heat pump systems pull heat from outdoor air and transfer it to a hydronic loop — running radiators, underfloor, fan coils and a domestic hot water cylinder from a single all-electric unit. This is the engineering page: how they work, what's inside, and how we design them into a complete home system.
A refrigeration cycle in reverse — moving heat, not creating it
Unlike a gas boiler that burns fuel to generate heat, an air-to-water heat pump moves existing heat from the outside air into your home. Even at 2°C outside, there's enough thermal energy in the air for a heat pump to extract and concentrate — using only the electricity needed to run the compressor and fan.
This is why heat pumps achieve coefficients of performance (COP) between 3 and 5 — for every 1 kWh of electricity in, you get 3–5 kWh of heat out. No combustion process can match that.
What's inside an air-to-water system
The system has four core components — an outdoor heat pump unit, an indoor hydraulic module, a domestic hot water cylinder, and the hydronic emitters around the home. Each plays a distinct role and can be sized independently to match the building's heat load and water demand.
Outdoor heat pump unit
The heart of the system. Contains the compressor, fan, evaporator coil and expansion valve. Sized in kW to match the home's heat-loss calculation. Sited outdoors on a slab or wall bracket with airflow clearance.
Indoor hydraulic module
The bridge between refrigerant and water. Contains the heat exchanger that transfers heat into the hydronic loop, plus the circulation pump, controls, expansion vessel and pressure gauges. Typically wall-mounted in a utility or plant room.
Hot water cylinder
An insulated tank where the heat pump heats domestic water for showers, taps and appliances. Sized 170–300L+ depending on bedroom count. Some systems use a hybrid setup with electric boost element for peak demand.
Hydronic emitters
The heat delivery surfaces inside the home — radiators, underfloor loops, fan coil units or trench convectors. The system can be designed with mixed emitter types per zone, all running from the same heat pump.
Not every emitter pairs equally — here's how they rank
Heat pumps are most efficient at lower flow temperatures (35–45°C). The lower the temperature your emitters need, the better the COP. This is the single most important design decision — and where retrofits often need adjustment.
Underfloor heating
IdealRuns at 30–40°C flow temperatures — the lowest in the system. Heat pump operates at its highest COP. Best-in-class pairing for new builds.
Oversized radiators
GoodStandard radiator sizing assumes 70°C+ flow. Heat pumps need radiators sized for ~50°C flow — usually larger or higher-output panels. New installs work well; some retrofits need radiator upgrades.
Trench convectors
GoodDesigned for low-temperature operation, so they pair efficiently with heat pumps. The convective airflow helps even at 45°C flow.
Fan coil units
GoodForced convection means they work well at moderate flow temperatures and can provide both heating and cooling. Good for retrofit zones with no underfloor option.
Existing high-temp radiators
ConsiderOlder radiator systems designed for boiler flow temperatures (70°C+) can run on a heat pump, but COP drops significantly. We assess on a case-by-case basis — often the answer is replacing or supplementing some panels.
Domestic hot water
IdealThe same heat pump handles your hot water cylinder. Most systems pre-program a daily cycle to 55–60°C with a periodic anti-legionella boost — all rebate-eligible under the Victorian Energy Upgrades program.
What needs to happen during the design phase
Air-to-water systems aren't a swap-in replacement for a gas boiler — they're a whole-of-system design decision. Get the design right and you'll have an efficient, all-electric home running for decades. Get it wrong and you'll have a heat pump struggling at high flow temperatures, dragging the COP down to gas-equivalent running costs.
We work with architects, builders and homeowners through the design phase to specify the right capacity, emitter mix and electrical infrastructure before construction starts.
Brief us on your projectDesign checklist
Air-to-water questions answered
How is this different from the electric heating page?
The electric heating page is the homeowner-buyer view — should you switch from gas, what does it cost, what are the rebates. This page is the engineering view — how the system actually works, what's inside it, and how to design it properly. Same product, different angle. Both worth reading if you're making the decision.
How well does it perform in Melbourne winters?
Very well. Modern air-to-water heat pumps maintain useful output down to about -10°C, which is well below anything Melbourne sees. Performance does taper as outdoor temperatures drop — a unit running at COP 4.5 in mild weather might drop to COP 2.5 on a 2°C morning. Still far better than direct electric or gas, but worth understanding when comparing manufacturer specs.
What size heat pump does my home need?
We do a heat-loss calculation as part of the design — it factors in the building envelope, insulation, glazing, orientation and target indoor temperature. Typical Melbourne homes land between 6–14 kW for heating, with hot water demand layered on top. Undersizing leads to poor winter performance; oversizing wastes capital and reduces efficiency. There's no shortcut to proper calculation.
Can it cool the house in summer?
Yes — by running the refrigeration cycle in reverse, the heat pump can chill water to circulate through fan coils, certain underfloor systems, and trench convectors for gentle cooling. It's not as powerful as a dedicated split or ducted AC system, so for hot Melbourne summers we often pair air-to-water with a separate cooling system. We'll walk through the trade-offs during design.
How noisy is the outdoor unit?
Modern units run between 40–55 dB at one metre — quieter than a fridge, comparable to background conversation. Siting matters: against a solid wall amplifies sound, while open positioning and rubber anti-vibration mounts minimise it. Council noise regulations apply for boundary distances and we factor those into the site assessment.
What about refrigerants — environmental impact?
Modern air-to-water heat pumps use R32 or R290 (propane) refrigerants with much lower global warming potential than older R410A systems. R290 in particular has a GWP of about 3 (compared to 2,088 for R410A) and is becoming the standard for new residential heat pumps.
What's the typical lifespan?
15–20 years with regular servicing. The compressor is the wear component — most modern units use inverter-driven variable-speed compressors that ramp up and down rather than cycling on/off, which extends life significantly. Annual maintenance is recommended to keep the heat exchanger clean and the refrigerant charge correct.
What rebates apply?
Victorian Energy Upgrades (VEU) rebates apply to heat pump hot water systems. Hydronic heat pump installations may also qualify depending on the configuration. See our heat pump rebates page for the current scheme details.
Designing an air-to-water system for a new build or major reno?
Book a system assessment and we'll work through capacity sizing, emitter selection, hot water demand, electrical infrastructure and rebate eligibility — all in one conversation with your architect or builder included if needed.